This invention generally relates to angular displacement devices, and more specifically to a high torque rotary electromagnetic actuator of the type generally used on miniature electromagnetic indicators.
Miniature rotary electromagnetic actuator indicators are well known in the art and are utilized to provide various types of indications. Most frequently, such indicators are used to provide changes in conditions or states, and are used very frequently, because of their size and small power consumption, in aircraft and other vehicles.
The miniature rotary electromagnetic rotary indicators of the type under discussion advantageously have the following properties. Firstly, it is important that the actuators develop sufficient torque to permit the rotary movement of an indicator, such as a flag or the like, which is typically attached to the shaft of the actuator. Because of the uses to which the actuators are typically put, it is imperative that the actuators be reliable and provide the desired indications or changes in indications or over wide ranges of applied signals or power supply voltages. It is also desirable that the actuators have well defined end positions, so that a positive indication can be provided to the viewer and there is no ambiguity as to the condition which the actuator monitors.
As a result of the above requirements, which are among the more important ones to be taken into consideration in the design and manufacture of such actuators, the tolerances of manufacture are frequently critical, this resulting in high manufacturing costs. The problems associated with miniature rotary actuators of the type being considered is compounded by the fact that the dimensions of such actuators are very small to begin with, so that the tolerances are even tighter and the use of special tools must be employed to shape or form the various parts which make up the actuator. This further increases the manufacturing costs.
The desirable high torque outputs, over the entire operative range of rotation of the actuators, are a function of the magnitude or strength of the magnetic fields which can be developed. The strength of the magnetic field, on the other hand, is a function, among other things, of the size of the coil or the number of turns which can be employed in the actuator, and the reluctance of the magnetic circuit. The magnetic reluctance, in turn, is a function of the magnetic materials used, as well as the air gaps formed in the magnetic circuit. The size of the coil is essentially fixed once the size of the miniature actuator is selected. Only so many turns can be used in such an actuator, and still provide sufficiently low coil resistance so as to permit the necessary current levels to flow therethrough. However, in the prior art constructions, the actuator reluctances have not been minimized and, therefore, the torques have not been optimized.
An angular displacement solenoid is disclosed in U.S. Pat. No. 3,221,191, issued on Nov. 30, 1965, discloses a rotor and stator construction in which the rotor is rotatably mounted on the central portion of an E-shaped stator. However that patent teaches the use of a channel-shaped hub member affixed to the shaft on which the rotor is mounted and which is made of a non-magnetic material. Being non-magnetic, the hub presents a high reluctance to the axial components of the magnetic flux, this decreasing the overall strength of the magnetic field in the solenoid with a resultant decrease in the available torque output. In devices exemplified by U.S. Pat. No. 3,311,859, the core has a flange at one end which mates with a cylindrical stator and, due to the large diameter of the mating surfaces, the air gap between the core and the stator is increased considerably, this materially increasing the reluctance in the path of the magnetic flux.
The rotary electromagnetic actuators disclosed in U.S. Pat. Nos. 3,234,436 and 3,289,133 show unitary or single piece construction stators. However, even if such single piece stators could be made, this would be highly impractical and costly to manufacture because it would require forming the stator, which has a substantially E-shaped cross-section, from solid stock material. By virtue of the generally small overall dimensions of such miniature rotary actuators, as suggested above, this is a difficult, time consuming and expensive task. Additionally, such single piece rotors of the types disclosed in these patents would, of necessity, have to be made from a relatively hard material such as cold rolled steel, which would permit the removal of the interior of the stator by appropriate machining steps. However, cold rolled steel or similar materials do not have the high permeabilities of other types of magnetic irons which, however, are not suitable for use in the manufacture of stators of the type shown in these two patents.
The rotors disclosed in the above patents are generally provided with counterbores for receiving a portion of a stator core, and are provided with coaxial hub portions which extend beyond the region of the counterbores. However, the hub and counterbore relationship in such rotors result in generally very small cross-sections between the hub and the rotor poles, this increasing the magnetic reluctance and magnetic flux losses with attendant decreases in output torque.